The invention relates to improvements made to convertible tilt-rotor aircraft which can travel in airplane mode and in helicopter mode, and of the type comprising at least one fuselage, at least one empennage with at least one stabilizer, a fixed wing structure comprising at least two wings extending laterally on each side of said fuselage and, in helicopter mode, a rotary wing structure comprising at least two rotors, each of which is supported and driven in rotation by a respective one of two drive nacelles each supported by a respective one of the two fixed wings extending from the fuselage as far as the corresponding nacelle, each rotor being mounted so that it can tilt with at least one front part, which supports said rotor, of the corresponding nacelle on the corresponding fixed wing and about an axis of tilt which is roughly transversal with respect to the fuselage, so as to switch between helicopter mode and airplane mode, in which mode the rotors act as propellers.
As is known, these convertible aircraft can operate in helicopter mode or configuration, particularly for landings and take-offs, during which the rotors rotate above the fixed wings, about axes that are roughly vertical so as to provide the aircraft with lift, and in airplane mode or configuration, in which the rotors are tilted with respect to the fixed wings so as to operate as propellers.
Each rotor has its shaft connected by a respective transmission to a respective engine, the transmission and the engine being housed in the corresponding nacelle supported by the corresponding fixed wing, an interconnecting shaft connecting the two transmissions, so that the two rotors can be driven in rotation by either one of the two engines, should the other engine fail.
U.S. Pat. No. 5,054,716 describes a first example of a convertible aircraft of this type, in which each of the rotors, together with its operating means, the corresponding engine and the corresponding transmission, constitutes a tilting assembly housed, with the exception of the rotor blades and hub, in a nacelle mounted so that the entire thing can pivot, cantilever fashion at the tip of a corresponding fixed wing.
An architecture such as this has numerous disadvantages, mentioned in patent application FR 99 03735, which describes another architecture of a convertible aircraft of this type, overcoming the aforementioned drawbacks and in which each transmission comprises a front reduction gear assembly, driving the rotor in rotation, and a rear reduction gear assembly engaged with the corresponding front reduction gear assembly and connected to the corresponding engine, and to the interconnecting shaft connecting the two transmissions. Each of the two nacelles is articulated and comprises a front part, mounted so that it can tilt about the axis of tilting on a fixed rear nacelle part fixed to the corresponding fixed wing and in which rear nacelle part the corresponding engine and at least part of the rear reduction gear assembly of the corresponding transmission are housed. The front reduction gear assembly and the corresponding rotor shaft are housed in the tilting front nacelle part and are mounted so that they can tilt with said front part with respect to said rear part and said corresponding wing.
Whatever their architecturexe2x80x94nacelles which tilt in full with the rotors, or articulated nacelles only the front parts of which tilt with the rotors with respect to the fixed wingsxe2x80x94the convertible aircrafts of the aforementioned type present, in terms of vibration control, new problems which are far removed from those presented by helicopters. Specifically, the special architecture of convertible aircraft, with rotors and possibly engines that can tilt at the wing tips, makes it very difficult to filter vibrations by inserting anti-resonant elements as is done on helicopters.
By contrast, the use of vibration filtration systems using resonators or force generators driven by computers is known, but these systems generally involve spring-mass assemblies which are very penalizing in terms of weight.
Furthermore, in a convertible aircraft of the aforementioned type, roll control in airplane mode is normally provided by the use of orientable command and/or control surfaces each mounted to pivot about an axis substantially transversal to the aircraft, along the trailing edge of each of the fixed wings, and these orientable surfaces are also used to provide the aircraft with additional lift at low speed in airplane mode and to reduce the offsetting power of the wing in helicopter mode.
As roll control requires swift dynamics, each fixed wing has a relatively high number of such command and/or control surfaces along its trailing edge, and these orientable surfaces need to be able to be commanded by a great many activators and may be connected by complex mechanical links allowing, in particular, negative turns (upward) and asymmetric turns of these surfaces across the two wings, while at the same time limiting the risk of a runaway roll situation associated with the asymmetric turnings of these orientable surfaces which are known as flaperons because they can perform the functions of flaps and of ailerons, and therefore of lift-augmenting and warping control surfaces. U.S. Pat. No. 5,094,412 describes means of complex structure and complex control of such flaperons, also known as elevons, for a convertible aircraft according to the aforementioned U.S. Pat. No. 5,054,716.
The problem underlying the invention is that of improving convertible aircraft of the aforementioned type mainly in terms of the active control of vibrations, while at the same time avoiding the addition of significant additional masses for essentially countering the excitations caused by the rotors and, secondarily, of making such an aircraft easier to control in terms of roll in airplane mode.
In this secondary objective, these improvements aim to allow simplifications to the maneuvering and command means for controlling the aircraft in terms of roll in airplane mode and preferably, at the same time, a simplification of the command and maneuvering means performing the functions of augmenting lift and reducing offsetting power.
The invention also relates to improvements made to the convertible aircraft of the aforementioned type to give them the ability to land in airplane mode (without converting beforehand from airplane configuration to helicopter configuration). This possibility makes it possible to reduce the critical nature of the rotor tilt mechanisms in that landing remains possible, without damage to the aircraft, whatever the position of the rotors.
Furthermore, if both engines fail in airplane mode, it is advantageous, particularly from the safety point of view, to be able to make a descent in airplane mode and in gliding flight and to land without having to tilt the rotors into helicopter mode before setting down, hence reducing the workload on the crew.
However, the ability of a convertible to land in airplane mode is usually accompanied by a reduction in the size of the rotors, and sometimes in the replacement of the rotors with airscrews, as is the case in a third architecture of convertible aircraft known as the tilt wing aircraft, in which the aircraft wings pivot in full or in part about the axis of tilting with the nacelles that they support.
Now, reducing the size of the rotors has known consequences on the performance of an aircraft of the VTOL (Vertical Take-Off and Landing) type. These consequences are, in particular:
degradation in the performance in hovering flight and at low speed, because the lift effectiveness of a rotor decreases rapidly with its size, thus eliminating the apparent gain achieved by effacing the wings under the rotors in embodiments with wings tilting with the nacelles and rotors, for example,
increasing the external noise, which is associated with the increase in the load at the rotor disks (maximum mass divided by the area of the rotor disks), and
degradation in the ability to windmill or autorotate which is associated with the increase in the load at the rotor disks.
The second problem underlying the invention is that of overcoming these drawbacks by the use of rotors of relatively large size, sized to optimize performance in hovering flight, with cyclic pitch control (which does not generally exist in tilt wing embodiments) and collective pitch control for good convertible-aircraft behavior in helicopter mode and during conversion, such large-sized rotors being tiltable with respect to fixed wings, so as to keep the advantages inherent in this type of wing structure, particularly so as to limit the drag in forward flight in helicopter mode (to improve performance on take-off in the event of an engine failure) with respect to a completely or partially tilting wing, and which allows good behavior during conversion. In addition, the wing structure of the convertible aircraft is configured to allow landing in airplane mode in spite of the presence of relatively large-sized rotors.
With the effect of solving the first problem underlying the invention and as explained hereinabove, the convertible aircraft according to the invention is characterized in that each fixed wing is extended, substantially in the direction of its span and toward the outboard side of the corresponding nacelle with respect to the fuselage, by at least one outboard wing portion, at least part of which pivots, independently of the rotor and of at least the front part of the corresponding nacelle, about an axis of articulation substantially transversal to the aircraft and constitutes an orientable command and/or control surface, whose pivotings about the axis of articulation are commanded, at least at a frequency of the order of Kbxcexa9, where b and xcexa9 are, respectively, the number of blades and the frequency of rotation of each rotor, and K is a whole number at least equal to 1, by at least one driven actuator so as to at least attenuate, at the fuselage, the fixed wings and the empennage(s) and stabilizer(s), at least the vibrations generated naturally by the rotation of each rotor.
Aside from controlling the vibrations generated by the rotors in normal operation, or natural vibrations inherent in the rotors and generated in rotating axis, the orientable and outboard wing tip command and/or control surfaces can be used to provide active control of the vibrations produced by either or both of the phenomena of whirl flutter and tail shake, the former of which, it will be recalled, is an aeroelastic instability arising from the looping between a rotor and the corresponding wing at high speed in airplane mode, while the latter corresponds to vibrations of the tail boom or of the rear parts of the nacelles and of the fuselage of a convertible aircraft, said tail boom or rear parts of nacelles and of the fuselage being excited by the wash of the rotors at frequencies of a few hertz and often close to 4 Hz. This active control makes it possible to reduce the stiffness, and therefore the mass, of each wing for a given maximum speed and/or to increase the maximum speed of the aircraft.
To this end, the pivoting of the orientable command and/or control surface is transiently commanded by the driven actuator at a frequency below Kbxcexa9 and of the order of 4 to 6 Hz so as to counter the whirl flutter phenomenon, or of the order of a few Hz, generally of the order of 4 Hz, so as to counter the tail shake phenomenon.
When integrated into an active anti-vibration system, the outboard orientable command and/or control surfaces, arranged as elevons operating as little driven ailerons or active aerodynamic flaps, situated at the wing tips, generate aerodynamic forces which are used to counter the excitations generated by the rotors, avoiding the addition of additional masses.
A convertible aircraft of the type set out hereinabove is thus equipped with a self-adaptive anti-vibration system based on the outboard orientable command and/or control surfaces or outboard elevons operating as driven ailerons and/or as active aerodynamic flaps at the wing tips, and the turn angle, and therefore incidence of which is driven by at least one computer commanding at least one maneuvering actuator, first actuator being controlled at the frequency bxcexa9, any second actuator being controlled at the frequency 2 bxcexa9, any third actuator being controlled at the frequency 3 bxcexa9, etc., so as to generate aerodynamic forces directed against the excitation forces of the rotors and thus making it possible to minimize the level of vibration in the fuselage, the empennage(s) and stabilizer(s), the fixed wings and any fixed rear parts of the aircraft nacelles that there might be, this anti-vibration system being particularly well suited to operation in airplane mode.
To this end, and advantageously, the actuator is an excitation ram slaved in movement, maneuvering the orientable command and/or control surface against the action of static and dynamic tuning elastic means and driven automatically by at least one active and self-adaptive vibration control computer which drives the ram on the basis of signals received from sensors, particularly load, accelerometer and gyrometer sensors, arranged at least at predetermined points on the fuselage and/or the rotors and/or the empennage(s) and stabilizer(s), in particular.
Advantageously, the elastic means absorb the static forces of the orientable surface and, in dynamic terms, their stiffness is coupled to the inertia of the moving assembly comprising at least said orientable surface and the moving parts of the ram so as to create a second-order resonant system, the resonant frequency of the moving assembly being tuned to the excitation frequency of the ram, which makes it possible to considerably reduce the control forces, and therefore the size of the ram.
In practice, the moving assembly has a resonant frequency       f    =                  1                  2          ⁢                      xe2x80x83                    ⁢          π                    ⁢                        k          I                      ,
where k is the stiffness of the elastic means (25) and I is the inertia of the moving assembly and the excitation frequency of the ram is normally tuned to bxcexa9 such that bxcexa9 is substantially equal to f.
When the two rotors of the convertible aircraft are three-bladed rotors, and given the nominal rotational speed of the rotors, the excitation frequency of the ram is normally tuned substantially to a frequency of about 20 Hz.
In addition, in order to counter the phenomena of tail shake and whirl flutter, the excitation frequency of the ram is advantageously transiently tuned to a frequency of about 4 Hz to about 6 Hz when said sensors detect signals that bear witness to at least one of these two phenomena, then, once the phenomenon has been attenuated or has disappeared, the excitation frequency of the ram is tuned back to substantially the frequency bxcexa9.
Such an outboard orientable surface (outboard of a nacelle) can also receive a differential command, with respect to the orientable surface outboard of the other nacelle, and operate as an aileron commanding warping and allowing the aircraft to be controlled in terms of roll in airplane mode, with the required swift dynamics, the roll command afforded by the outboard orientable surfaces thus being decoupled from the lift-augmenting and lift reduction functions that can be carried out, with slower dynamics, by other inboard (between the nacelles and the fuselage) orientable command and/or control surfaces on the trailing edges of the fixed wings.
In this alternative form, the excitation ram of each elevon or outboard orientable surface is also driveable by pilot controls (actuated by the crew of the convertible aircraft), particularly warp controls. In this case, command of the excitation ram by the vibration control computer is neutralized while the excitation ram is being commanded by the pilot controls.
EP-0 416 590 and U.S. Pat. No. 3,666,209 disclose convertible aircraft the wing structure of which comprises aerodynamic lift-creating and pivoting surfaces outboard (along the wing span) of drive nacelles and of a fixed inboard wing portion. However, each drive nacelle pivots with a wing part about an axis of pivoting, which means that these convertibles have the third of the aforementioned convertible aircraft architectures known as the tilt wing architecture and their pivoting and outboard aerodynamic surfaces are intended to correct variations in attitude of the aircraft about its center of gravity and are therefore control surfaces activated by flight controls situated in the cockpit. Each flight control or pilot control allows the aircraft to be moved about one of its axes of roll, pitch and yaw.
Depending on the case, these moving surfaces can be likened to ailerons allowing warping (rotation about the axis of roll) in flight in airplane mode or a movement of yaw in vertical flight in helicopter mode.
According to the present invention, the pivoting parts of the outboard wing structure portions are, unlike in EP-0 416 590 and U.S. Pat. No. 3,666,209, elevons, the functions of which have been defined hereinabove and which are self-driven to minimize, in the structure of the aircraft, vibrations which are at least of the aforementioned three types: vibrations generated by the rotors in normal operation, and by the phenomena of tail shake and whirl flutter.
The outboard orientable surfaces according to the invention can also be used to reduce the rates of descent of the aircraft with the rotors windmilling or autorotating (in the event of a failure of the two engines), contributing to the lift of the aircraft if these orientable outboard surfaces are directed into the wind.
Furthermore, the presence of such outboard orientable surfaces has the impact of increasing the aerodynamic elongation of the wings and therefore of reducing the induced drag, thus improving performance in airplane mode in a climb, in cruising flight and in fineness, hence giving a lower rate of descent in unpowered flight (engine failure).
By analogy with the embodiment of the conventional ailerons and flaps, the pivoting part or orientable command and/or control surface of each outboard wing structure portion may be a pivoting trailing edge elevon of a fixed and outboard wing portion, which thus constitutes the tip of the corresponding fixed wing, beyond the corresponding nacelle. However, in a second embodiment, each outboard wing structure portion may be an outboard wing part that is entirely pivoting about the axis of articulation so that the entirety of the wing structure part outboard of a nacelle can be arranged as an elevon pivoting about its axis of articulation with respect to the adjacent nacelle and with respect to the corresponding fixed wing.
With a view to solving the second problem underlying the invention and as is set out hereinabove, the convertible aircraft according to the invention is such that its fixed wings are high wings secured to the upper part of the fuselage, to keep the nacelles and therefore the rotors at a sufficient height, guaranteeing a minimum ground clearance of the rotors to allow landing in airplane mode, this ground clearance being increased and/or the diameter of the rotors being increased if the high wings are raised with respect to the upper part of the fuselage.
However, advantageously in addition, the fixed high wings have an upward dihedral angle (positive dihedral angle) between the fuselage and the nacelles, which, at the same time, makes it possible to increase the ground clearance and/or the diameter of the rotors still further and makes it possible to limit the drag penalty due to the raised position of the fixed wings above the fuselage.
Fixed wings which are raised with respect to the fuselage and with an upward dihedral angle undeniably improve the landing capability with the rotors in airplane mode.
Outboard of the nacelles, the outboard wing structure portions comprising the outboard orientable command and/or control surfaces, or arranged as such outboard orientable surfaces, may also have a positive (upward) or zero (substantially horizontal) dihedral angle, but advantageously, in order to at least partially compensate for any disadvantageous aerodynamic effects of the positive dihedral angle of the fixed wings, the outboard wing structure portions may have a negative (downward) dihedral angle so that the fixed wing structure of the aircraft is substantially in the form of gull wings.
It should be noted that the characteristics relating to the raised fixed high wings with an upward dihedral angle and which are possibly extended, outboard of the nacelles, by outboard wing structure portions with a positive, zero or negative dihedral angle, can be used on a convertible aircraft of the type set out hereinabove independently of the other characteristics set out hereinabove and relating to the structure, the arrangement, the maneuvering and the control of the outboard orientable command and/or control surfaces of the convertible aircraft, and vice versa. Specifically, such outboard orientable surfaces may equip the wing tips of a fixed wing structure of a convertible aircraft, the fixed wings of which are not high wings nor are they raised nor do they have a positive dihedral angle.